The attitude and heading reference system (AHRS) for an aircraft is typically based on a complementary filtering algorithm. In order to increase solution accuracy, statistical filters, typically based on first-order approximations (e.g., extended Kalman filters), are commonly used. A drawback of this approach is in the polar regions, where the heading (e.g., magnetic aiding) is not sufficient to maintain the filter in its linear region.
At the polar regions, the horizontal component of the magnetic field vector becomes very small (or zero), making heading determination a challenging task. Furthermore, in the immediate vicinity of the geographic pole, very (or infinitely) high vertical angular rates are required to keep a vehicle locally-levelled (tangent) coordinate system aligned to the north. However, in such cases, the AHRS must be able to provide the user with reliable estimates of the attitude (roll and pitch angles) even though the output from the heading source is unreliable or completely missing.